This invention relates broadly to means for transferring forces imposed on load bearing portions of artificial joints to bone in humans and animals. While the present invention is applicable for use with implants of various types and in numerous applications in human and animal joints, it will be described herein for purposes of example only specifically as adapted for use in transferring the load on the femoral head replacement of a total hip joint prosthesis such as the Charnley type, to the femur. Although hip prostheses will be used for illustrative purposes only, features of the invention will be stated in a generic form so that they are applicable to all joint prostheses and to the geometric and biomedical properties of all animal and human joints.
One of the fundamental problems that has been encountered in the development of prosthetic replacements for the major joints of the body is the means utilized for attaching the implant to bone. Various forms of joint prostheses currently used are held in place in the body commonly by one of three methods: (1) a stem which is impacted into the medullary canal of the bone; (2) a stem which is fixed mechanically by internal fixation as provided by bone screws, pins, or the like; and (3) a stem which is fixed by bone cements, grouts or adhesives such as polymethylmethacrylate which is polymerized in situ and serves as a cement or filler between the stem of the prosthesis and the bone.
Each of these methods currently in use has presented problems that can lead to failure of the arthroplasty. The problems are as follows:
Devices that are impacted into the medullary cavity of the bone are held in place by the surrounding bone. The actual surface of contact between the prosthesis and the bone may be very small and areas of stress concentration will frequently occur. Clinical observations have been reported associating loosening of the implant (due principally to bone resorption) and joint pain.
Historically, when mechanical internal fixation has been used, clinicians take exception to the difficulties of inserting the implant and to the long term inadequacy of the fixation of the implant to bone which results in patient discomfort.
Polymethylmethacrylate, when polymerized in situ, will initially secure stems of total joint prostheses firmly in place allowing the patient to use the joint without pain. However, there is growing clinical evidence concerning the long term efficacy of this form of fixation. Three kinds of the most frequent failure modes of polymethylmethacrylate fixation which are associated with prosthesis stem failure are: (1) improper (varus) placement of the stem at the time of surgery; (2) breakdown, cracking and dislocation of the cement; and (3) resorption of bone tissue with the associated loosening of the cement-bone fixation.
In each instance, a prosthesis stem fails because of the excessive loads on the stem due to the change in the support and transfer mechanism between stem, cement and bone. Tensile stressses in the stem, e.g., the lateral surface of a hip prosthesis stem, due to the bending moment imposed by a patient's weight or muscle contractions, will lead to fatigue failure under repetitive cyclic loading. Typically, a fatigue crack will initiate in the region subjected to high tensile stress and will propagate through the stem cross-section by slow crack growth until the metal in the remaining cross-section is insufficient to support the loads, thereafter rapid cracking will occur through this remaining cross-section. Also, ordinary state-of-the-art difficulties in manufacture cause discontinuity in the metals such as voids, inclusions, notches and scratches, and accordingly these areas are sites for potential crack initiation.
Attempts have been made to reduce the stress level in the stems of various prostheses by design modification. For example, the neck length and neck shaft angle of the hip prostheses have been modified. A simple way to decrease the stress in stems is to increase the section modulus of the stem. Because this provides a bulky, heavy, more rigid prosthesis, problems of insertion are encountered as the section modulus is increased.
We believe that it is not ideal to have a relatively rigid stem and in fact the preferred way to transfer the load from bone to the prosthesis and then to bone is by using a stem with lower relative rigidity, directly contrary to teachings of the prior art.
The more directly load can be transferred from bone to the load bearing portion of the implant to bone, the more natural will be the bone's reaction thereto, therefore less resorption will occur, and more bone material will be retained. The lower rigidity stem(s) facilitate transfer of the load out of and then into the bone more directly, rather than carrying load down (or up) to the tip of the prosthesis before transferring it to bone. In order to utilize the presently available stems, especially those of lower relative rigidity, more effectively, there must be a change in the mechanical properties of the presently used method to interface the stems with bone. The present invention (a device) is inserted between the stem(s) and bone together with polymethylmethacryate or similar cement materials and is capable of deforming and efficiently transferring the load into the bone. The device produces a new interface system which will not break up with presently available (or less rigid) stem(s), and provides a system that has increased strength and can undergo larger deflections than is the case with the simple use of polymethylmethacryalte alone.
Until now, emphasis has been placed on providing extra strength in the stems of prostheses, usually in the form of extra thickness. The present invention overcomes the problems prevalent in the prior art, specifically in the use of polymethylmethacryalte which has a low modulus, low strength and is extremely brittle. It of course has been noted that when the polymethylmethacrylate fails, the implant often fails.